U.S. Pat. No. 5,223,117 relates to self-assembly microelectrodes used in electrochemical sensors. The microelectrodes (which are the working electrode in the sensors) are modified with monolayer coverages of reference and indicator molecules, with both chemically sensitive redox materials and chemically insensitive redox materials being present on the same electrode. It is necessary for the microelectrodes to be significantly smaller than the counterelectrodes, for example the counter electrode area must be at least 102 to 103 times the working electrode area. An advantage of such a small working electrode with an internal reference is that the sensor is minimally invasive, and can therefore be used in biomedical sensing.
UK Patent Application No. 2 391 314 describes electrochemical sensors for measuring the amount of hydrogen sulphide or thiols in a fluid. The sensor comprises a precursor and reaction solution which, together with the hydrogen sulphide or thiols, create a redox reaction. The current produced by this redox reaction is dependent upon the concentration of hydrogen sulphide or thiols. The sensors described in this document are for use in downhole applications, i.e. to extend down boreholes during a drilling operation. Given the size restrictions on apparatus which must extend into a borehole, the sensors must be relatively small.
Carbon-based electrode materials have been in use for many decades. The main forms of carbon in common use are glassy carbon, carbon fibres, carbon black, graphite, carbon paste and carbon epoxy electrode. Carbon is an attractive electrode material as it is relatively chemically inert yet it has a high surface activity and a wide operational potential window (ca. −1.0 V to +1.0 V vs. the saturated calomel electrode in aqueous solution).
However, there remains a need for more robust, reagentless sensors that can provide accurate results in hostile environments or “dirty” media such as effluents or sewage. Furthermore, there is a requirement for sensors to be used under various conditions, such as at temperatures above room temperature. There is renewed interest in developing sensors capable of measuring pH accurately at elevated temperatures. The present invention aims to address these issues.